It has already been proposed, as the discussion below will show, to provide silicon-carbide articles, bodies or shapes, especially tubular articles, by processes in which a preshaped body containing carbon is subjected to chemical reaction with silicon to form the silicon carbide in situ.
Silicon carbide bodies or articles are used in a variety of nuclear-reactor applications, e.g. as enclosures for nuclear reactor fuels or breeder substances, as ducts or as other passage-defining elements or containers.
In the past, shaped bodies could be treated to form a silicon carbide layer of depths or thicknesses of 7 to 10 mm from the surface at which the siliconization treatment was effected.
In other words, in earlier processes in which transformation of carbon of the body to silicon carbide proceeded inwardly from the surface at which the treatment was applied to the indicated depth transforming all or at least a predominant portion of the carbon in this region to silicon carbide.
The body usually consisted of carbon, a mixture of carbon and silicon carbide powder, or mixtures of either with further refractory materials which were also inert to silicon vapor. The silicon vapor was brought into contact with the body and penetrated the porous or gas-permeable structure and reacted with the carbon.
With earlier treatments of this type having a transformation depth of 7 to 10 mm from the treated surface, it was possible to fabricate articles having a maximum thickness of silicon carbide of about 20 mm. When larger thicknesses of the body were present, the transformation depth was at most 10 mm from each surface, thereby leaving a core of carbon-containing material which was unaffected by the siliconization process.
A number of processes for producing silicon-carbide bodies have been described in the literature and it has been proposed, for example, to hot press a granular or particulate material to produce the preform or shaped carbon-containing article (generally composed of carbon or a mixture and silicon carbide) and then to subject the shaped article to a chemical transformation, hereinafter referred to as siliconization.
For the siliconization step, the preform had to have a significant porosity which could be provided by forming a composition of the carbon-containing particulate (of particle size from say 50 to 100 microns) with a phenolformaldehyde resin powder. As described, for example, in German Pat. Nos. 2,133,044 and 2,360,982, the slurry or mass can be introduced with vibration into a mold and can then be hardened with pore generation by expansion of the vapors, vapor generation or the like and/or such that the entire body was carbonized by a subsequent cokefication treatment.
The resulting porous body consists predominantly of carbon and is especially effective as the starting article for a system of the present invention.
In the conventional siliconization processes, the carbon-containing porous body was introduced into a silicon-containing atmosphere either at ambient or standard pressure and a temperature of about 2000.degree. C. or at a subatmospheric pressure, generally of around 0.5 mbar and a temperature between 1500.degree. and 1700.degree. C. Both of these procedures were effectively used to transform at least surface regions of the carbon body to silicon carbide (see British Pat. No. 866,813 and Powder Metallurgy, Vol. 8, pp. 113 ff., 1961).
In German Patent document (Open Application-Offenlegungsschrift) DE-OS No. 2,239,971, graphite-containing silicon carbide bodies, intended to constitute antifriction materials, are produced by impregnating the porous preform composed of graphite with liquid silicon at a temperature of 1700.degree. C. to 2050.degree. C., by immersing the body in a silicon melt. Another system in which liquid silicon impregnation is used is described in "The Fabrication and Properties of Superbonded Silicon Carbide Bodies", TRC Report 2053 (UDC No. 661.665.1).
In German Patent document (Open Application-Offenlegungsschrift) DE-OS No. 2,256,326, a porous shaped body of .alpha.-silicon carbide and graphite is transformed in the presence of elemental silicon in vacuum or in an inert gas atmosphere at a temperature of 1400.degree. C. The silicon melts and penetrates the pores to form a body composed of .alpha. and .beta. silicon graphite with small residues of elemental silicon. This system differs from that of German Patent document No. 2,239,971 in that the porous body has less graphite and is siliconized at a lower temperature with joint heating of the body and the silicon. By contrast, a molten silicon bath must first be prepared in accordance with the German Patent document.
The German Patent document (Open Application-Offenglegungsschrift) DE-OS No. 2,439,930 describes a process in which an injection molding technique is used to produce bodies of silicon carbide powder with a continuous polymer matrix which, upon hardening, is subjected to pyrolysis at about 1000.degree. C. to produce a porous carbon-bonded body which is siliconized in an evacuated chamber at 1450.degree. to 1600.degree. C.
All of these systems have various disadvantages, ranging from the need for complex equipment and extreme operating parameters or inconvenient conditions, or are inconvenient because of the nature of the steps and preconditions such as the nature of the shaped body in which is to be siliconized. For example, in many cases the preparation and maintenance of a molten silicon bath is not convenient, the generation of silicon in a vapor phase may be undesirable, etc. Frequently, moreover, the prior art process is incapable of being carried out in a simple apparatus or does not result in a deep penetration of the siliconization treatment.